In the past, it has been problematic to coat high-carbon, non-enamel grade steel with porcelain enamel coatings due to defects that occur during firing of such coatings. This is especially problematic when attempting to coat the steel using a single firing. Such defects are often caused by what is known as the primary boiling effect which can be observed during firing of porcelain enamel coatings. The primary boiling effect is caused by gaseous products of hydration and combustion of the carbon in the high-carbon steel sheet. These gases consist of carbon monoxide, carbon dioxide, hydrocarbons and traces of nitrogen. The boiling gases cause defects in the finished enamel coating, such as blisters, bubbles, voids, pinholes, boil-outs, copperheads and other defects. Such defects adversely affect the strength, weather resistance, corrosion resistance and appearance of the coating.
Additionally, such defects on the surface of the enamel coating may cause spalling due to freezing and thawing action of water which enters into the defects at the enamel surface. This initial spall can then open up more bubbles or pores and through subsequent freeze/thaw cycles produces more spalling.
It is believed that during the firing of the glass directly to the steel at elevated temperatures, water or moisture from a number of sources reacts with the steel to form hydrogen gas. During the high-temperature firing process of the porcelain enamel, the hydrogen gas formed by this reaction penetrates into the steel. After firing, and as the enamel coated steel is cooled, the hydrogen, being less soluble in the cooler steel, is discharged from the steel and builds up a pressure beneath the solidified enamel coating. This is especially problematic in cases where all of the surfaces of the steel substrate are coated with the enamel. The result is that the coating pops off in small flecks known as "fish scales" due to the pressure of the trapped gas. Defects which are caused by the trapped hydrogen are known as hydrogen defects.
U.S. Pat. No. 2,940,865 issued to Sullivan discloses the use of a thin layer of nickelous oxide coated on the steel substrate prior to enameling with a single layer of enamel. The stated function of the nickelous oxide layer is to reduce hydrogen defects and increase the adherence of a single thick layer of porcelain enamel coating to a steel base. The '865 patent discloses it is believed that the thin coating of nickelous oxide reacts with the water during the firing process away from the steel surface. Atomic hydrogen that is produced by the reaction, rather than penetrating into the steel, will combine to form molecular hydrogen which will not penetrate into the steel. It is also believed that the nickelous oxide layer is porous and is believed to resist the progress of the atomic hydrogen from penetrating into the steel so that the atomic hydrogen will combine into molecular hydrogen rather than penetrate into the steel.
However, it has been found that, with the layer of nickelous oxide as disclosed in the '865 patent, boiling defects, especially pinhole defects, may still occur in the single layer of porcelain enamel coating. Discontinuities, such as pinholes, in the porcelain enamel coating can expose the steel substrate, thereby subjecting it to corrosion. Accordingly, if discontinuity defects are minimized, then coating performance improves.
It would be desirable to provide for a porcelain enamel coating that would allow for the coating of higher carbon steel and reduce the defects in a porcelain enamel coating that are caused by the boiling of gases and trapped hydrogen pressure upon the porcelain enamel coating, as well as provide for desirable coating characteristics. It would also be desirable to provide such a coating which could be prepared in a single firing.